The present invention relates to traffic shaping apparatuses and traffic shaping methods, and more particularly, to a traffic shaping apparatus and a traffic shaping method which effectively use a transmission frame rate in response to a change in transmission frame rates used by the users.
Before starting to use a public network (or public line) for communication, the user makes a contract with the administrator of the public network for a transmission frame rate, frame (packet) transfer priority, and others. After such a contract is exchanged between the user and the administrator of the public network, the user uses user terminals to start transmitting frames to the public network. A relay apparatus at the administrator side, positioned at the entrance of the public network, monitors the transmission frame rate of the user terminal, and if the contracted transmission frame rate is exceeded, lowers the frame priority of the violating user terminal or discards violating frames. This monitoring function of the public network is called a policing function or usage parameter control (UPC). Even if a frame is discarded during communication, the receiving terminal usually notices that the frame was discarded and requests the transmitting terminal to re-send the frame. Since the transmitting terminal re-sends the frame in response to this request, the finally received information has no omission. However, re-transmission may make a transfer delay very long, or resent frames may cause congestion in the network. Therefore, it is preferred that frame discarding not occur. It is necessary for a relay apparatus at the user side, which transmits frames to the public network, to control the transmission frame rate during frame transmission such that frames are not discarded in the public network. Such a function to control the transmission frame rate is called a traffic shaping function or simply a shaping function. A traffic shaping apparatus for implementing the traffic shaping function may be included in a relay apparatus directly connected to the public network and the user transmission terminal, such as those described above, or in a relay apparatus disposed at the exit of the public network to transmit frames to a private user network.
As such a shaping apparatus, an apparatus having a queue for high-priority packets and a queue for low-priority packets has been proposed. If there is no transmission-pending packet in a priority queue at the time of packet transmission from the priority queue, in order to effectively use the transmission frame rate, the shaping apparatus gives high priority to a transmission-pending packet in a non-priority queue and then sends it. Such a shaping apparatus is disclosed, for example, in EP Publication No. 1054544A2 (corresponding to Japanese Unexamined Patent Application Publication No. 2000-332787).